There are a variety of uses for dies that punch shaped holes and cut or form sheets from materials such as metal, cardboard and other stock. Dieboards which are slotted to receive and rigidly retain steel rule are in particular demand.
Dies have been described in previously issued U.S. patents. For example, U.S. Pat. No. 3,863,550 describes a die having a superimposed pair of metal or rigid plastic plates separated by an intermediate semirigid plastic material. The plates are coated with a light-sensitive compound to form a "resist layer" that is resistant to chemical etching materials such as nitric acid. A solubilizing agent removes the resist layer in appropriate areas indicated by a slotting template which is transferred onto the resist layer using photographic negatives. The dieboard is then slotted to receive rule by applying a chemical etching material. The superimposed metal or plastic plates of the dieboard described in this patent are expensive and chemical etching is relatively slow and costly.
Dieboards with a white finish, such as acrylic are known to facilitate projection or drawing of slotting templates onto the dieboard.
Because slotting can be accomplished more rapidly, lasers appear to be the fastest growing method of cutting dieboards. Lasers are currently being used to slot hardwood dieboards, such as of maple or birch. Due to their lack of dimensional stability, hardwood dieboards constitute the low performance and low cost end of the commercial market for dieboards. Lasers have also been used to cut plywood dieboards; however, the heat generated by the lasers often results in warping such dieboards.
Lasers have also been used to cut "PERMAPLEX" dieboards, which are made from a polyester-cellulose blend by EHV Weidmann of St. Johnsbury, Vt. PERMAPLEX boards are expensive. Also, these dieboards can only be slotted at a relatively slow rate using lasers.
Polyurea-cellulose composites are known in the art, but have not heretofore been recognized as being suitable for use in a dieboard structure. For example, U.S. Pat. No. 5,008,359, issued Apr. 16, 1991 to Frank Hunter and owned by the assignee of the present application describes such a polyurea composite material. This patent is incorporated herein by reference in its entirety and describes a polyurea-cellulose composite formed by impregnating cellulose sheet material with from about 8% to about 20% of a substantially uncatalyzed, polyisocyanate resin and thereafter curing this material under suitable conditions of moisture content, pressure and temperature.
The polyurea-cellulose composite described in the above patent has been used to form a sheathing panel as described in pending U.S. patent application entitled "A Multi-Functional Exterior Structural Foam Sheathing Panel," Ser. No. 07/680,810, filed Mar. 22, 1991. The sheathing panel comprises a foam core sheet of from one to four pounds per cubic foot density laminated with the polyurea-cellulose composite sheets. The sheathing panel application specifically calls for composite sheets of from about 8.times.10.sup.-3 to about 0.1 inches in thickness. This structure, because of its low density core, would not be suitable for a dieboard.
There is therefore a need for dimensionally stable, high-performance dieboards produced from inexpensive materials such as polyurea-cellulose composites. There is also a need for dieboards that can be slotted at a rapid rate, such as using lasers.